As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas- and light-barrier material, e.g. an aluminium foil or an ethyl vinyl alcohol (EVOH) foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. The web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which may be fed to a folding unit to form the finished packages.
More specifically, the pillow packs substantially comprise a main portion, and opposite top and bottom end portions tapering from the main portions towards respective top and bottom sealing bands which extends substantially orthogonal to the axis of the pack. In detail, each end portion is defined by a pair of respective trapezoidal walls which extend between main portion of the pack and the relative sealing band.
Each pillow pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin formed by respective sealing band; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by end portions of respective trapezoidal walls and by corresponding triangular portions which project from the main portion.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing band.
Packaging machines for producing packages of the above type are known, typically comprising:                an in-feed conveyor;        a folding unit receiving the pillow packs from the in-feed conveyor and adapted to fold these pillow packs to form the parallelepiped-shaped packages;        a transfer unit for transferring and up-ending sealed folded packages, which is arranged downstream from the folding unit and receives the sealed packages from the folding unit; and        an out-feed conveyor which receives folded packages from the transfer unit and moves them away from the packaging machine.        
Folding units are known, for example from EP-A-0887261 in the name of the same Applicant, which substantially comprise:                a chain conveyor for feeding packs continuously along a forming path from a supply station to an output station; and        first folding means and second folding means, which cooperate cyclically with each pack to flatten respective end portions of each pack and so fold respective fins onto end portions.        
In detail, the first folding means comprise a fixed guide member, which is positioned facing and at a distance from a conveying portion of the chain, and converge towards this conveying portion. The fixed guide member cooperates with bottom end portion of each pack to press it down flat towards the chain.
The second folding means comprise a pair of contrast elements defined by parallel elongated sections fitted to a fixed frame adjacent to respective opposite lateral edges of the chain conveyor. More precisely, the contrast elements cooperate in sliding manner with the top end portion of each pack to fold the top fin towards the main portion of the relative pack, and top flaps onto this main portion.
In this way, as the chain conveyor feeds the packs in a first direction, the relative top fins are folded onto the flattened main portion in a second direction, opposite to the first direction.
In detail, the top fins are folded onto the flattened main portion on the opposite side of a longitudinal sealing band.
Though efficient, packaging machines of the above type leave room for improvement.
In particular, a need is felt within the industry to render as controllable as possible the folding of the top fins onto the flattened main portion of the corresponding packs.
In particular, a need is felt within the industry to fold the top fin as coplanar as possible onto the flattened main portion.